PowerBook - Splitting current partition into 2..

Okay, so I want to dual-boot my G4 Powerbook (specs under avatar), with Ubuntu..

Now, I know how to split it using Disk Utility and have researched a bit and found this and have followed it. However, when I go to drag the bar to make the sizes different (I just want about 5gb for Ubuntu), I cannot drag the size-bar.

Any one know how I can partition my drive without wiping my current OS X installation clean? I'm currently on 10.4.11.